1. Field of the Invention
The present invention relates to wireless systems, and more particularly, to a wireless system for performing wireless communication according to W-CDMA (Wideband-Code Division Multiple Access).
2. Description of the Related Art
In recent years, a wireless communication scheme called HSDPA (High Speed Downlink Packet Access), which is based on W-CDMA technology, has been developed. With HSDPA, a wireless system is realized which employs adaptive modulation so that the modulation scheme may be switched from one to another at a base station in accordance with the radio wave receiving environment of a mobile terminal.
In HSDPA, higher-speed modulation schemes are used for mobile terminals in better receiving conditions, among those situated in an identical cell, thereby achieving high-speed downlink packet transmission. In addition to the switching of modulation schemes, a process is also performed such that packets are transmitted preferentially to those mobile terminals which are in good receiving conditions.
In order for a base station to recognize the receiving environments of mobile terminals, the procedure described below is followed. First, the base station sends out a pilot signal (common pilot signal) with a known carrier frequency, which is received by mobile terminals. On receiving the pilot signal, each mobile terminal measures the propagation environment (amount of interference) as the present receiving environment and notifies the base station of the propagation environment.
Based on the received information on the propagation environments, the base station selects mobile terminals which are in good receiving conditions in terms of interference amount, and then transmits a data channel with a transmission format using high-speed modulation or preferentially transmits a data channel to the selected mobile terminals.
More specifically, the propagation environment information is represented by a CQI (Channel Quality Indicator) obtained by converting the SIR (Signal-to-Interference Ratio (S/I)) of the pilot signal to a corresponding one of 30 different values, namely, “1” through “30”. For example, the CQI “1” is the smallest value of SIR and indicates that the reception quality of the receiving side is of the lowest level. The CQI “30” is the greatest value of SIR and indicates that the reception quality of the receiving side is of the highest level. FIG. 8 shows the correspondence between the CQI values and their applicable modulation schemes, or in other words, the correspondence between the CQI values and their applicable transmission formats (block sizes, modulation schemes).
On the other hand, the pilot signal is not separately transmitted to individual users; it is allocated to a certain code to be used in common among users. This makes it possible to effectively use the frequency band. Also, in order for the pilot signal to be used by all mobile terminals, the transmit power and spreading ratio of the pilot signal are set to large values.
As conventional techniques, there has been proposed a technique in which a mobile station measures the reception quality of a pilot signal, and during the setting of an uplink quality control channel via which quality information is transmitted to the base station, the mobile station starts to transmit the quality information to the base station at predetermined intervals (e.g., Unexamined Japanese Patent Publication No. 2003-199173 (paragraph nos. [0027] to [0031], FIG. 1)).
In cases where the interference amount is measured using the pilot signal at a mobile terminal located in a very good receiving environment, however, variation of the constellation points caused by phase noise or fading is so observable that the CQI fails to be correctly determined, because the transmit power and spreading ratio of the pilot signal are large.
FIG. 9 exemplifies SIR simulation results, wherein sampled symbols of a measurement signal are plotted on the constellation diagram and the tail of the thick arrow indicates the position where the symbol of the original measurement signal is located.
When the measurement signal is propagated with low transmit power and a small spreading ratio (like the data channel) in the simulation environment, the influence of phase noise and fading is small, and therefore, deterioration in the reception quality caused by such factors is also small.
Where such a measurement signal is received and its voltage value is measured, the measured voltage values are plotted as shown in FIG. 9. Specifically, symbols obtained by sampling the measurement signal at different sampling times are concentrated around the symbol of the original measurement signal, as indicated by Sa, because both the amplitude variation (variation in the distance from the origin to the symbol) and phase variation (rotational variation of the symbol with reference to the origin) with respect to the symbol of the original measurement signal are small, showing that no significant error is caused.
On the other hand, when the measurement signal is propagated with high transmit power and a large spreading ratio (like the pilot signal), phase noise and fading greatly affect the measurement signal and thus the reception quality is significantly lowered by these factors.
Where such a measurement signal is received and its voltage value is measured, the measured voltage values are plotted as indicated by Sb. Specifically, symbols obtained by sampling the measurement signal at different sampling times are located farther from the origin and also scattered in phase, as indicated by Sb, because both the amplitude variation and phase variation with respect to the symbol of the original measurement signal are large, proving that significant errors are caused.
FIG. 10 shows a CQI-SIR conversion table, wherein the vertical axis indicates SIR (dB) and the horizontal axis indicates CQI. A curve K1 shows the results of SIR simulation without phase noise, and a curve K2 shows the results of SIR simulation with phase noise.
When there is no phase noise, the SIR shows a nearly linear characteristic, as indicated by the curve K1. On the other hand, when there is phase noise, the SIR shows a different characteristic because error occurs as explained above with reference to FIG. 9. Specifically, the slope of the SIR characteristic begins to decline in the vicinity of the CQI “22”, as indicated by the curve K2 in FIG. 10.
Suppose that the SIR of the pilot signal (curve K2) is measured and found to be 26 dB. In this case, the CQI of the pilot signal is “28” but the CQI of the actual data channel (curve K1) corresponding to the SIR of 26 dB is “25”, showing that there is a difference between the CQI obtained by measuring the SIR of the pilot signal and that obtained by measuring the SIR of the data channel.
Namely, the CQI should originally indicate the reception state of the data channel. However, since in conventional systems, the CQI is obtained by measuring the SIR of the pilot signal which shows a propagation state different from that of the data channel and which is susceptible to phase noise and fading, a problem arises in that the obtained CQI does not exactly correspond to the actual reception state of the data channel. As a result, the base station fails to select a suitable transmission format for transmitting the data channel, causing throughput degradation.